This invention relates to a waveform shaping circuits used in the pulse generator circuit provided in all the fields of television transmitters and receivers, other video equipment, sequence controllers, etc.
A conventional waveform shaping circuit of the type referred to has comprised a voltage comparator having a negative input which is supplied with a reference voltage developed at a junction of two voltage dividing resistors which are serially connected across a source voltage and having a positive input which connected to capacitor which is charged by the source voltage through a charging resistor. The capacitor is connected via a discharging resistor to a collector electrode of an NPN type common emitter transistor having a base electrode which is connected to an input terminal through a base resistor.
A train of square input pulses is supplied to the input terminal so as to repeat the turn-on and -off of the transistor in an alternating manner. During the turn-off of the transistor, the capacitor is exponentially charged by the source voltage until one of the input pulses is supplied to the transistor so to turn the transistor on, thereby initiating the discharge of the capacitor. During the turn-on of the transistor, the voltage on the capacitor is exponentially discharged until that input pulse terminates so as to turn off the transistor. The voltage on the capacitor thus charged and discharged is supplied to a voltage comparator where it is compared with the reference voltage. The voltage comparator is responsive to a voltage on the capacitor which is equal to the reference voltage so as to produce a rising edge of a square output pulse at the output thereof and is also responsive to a voltage on the capacitor which is equal to the reference voltage so as to produce a falling edge of the square output pulse at the output thereof. Thus, the voltage comparator produces a square output pulse having a pulse width corresponding to a time interval for which the voltage on the capacitor is not less than the reference voltage.
The process as described above is repeated with each of the successive input pulses thereby resulting in a train of square output pulses.
Each of the square output pulses thus produced has rising and falling edges which are bilaterally unsymmetrical with respect to a time point where the rising edge of the square input pulse is developed. This is true in the case where the square output pulses are produced about the falling edges of the input pulses. In the conventional waveform shaping circuit, a time interval between the rising edge of the output pulse and the rising or falling edge of the input pulse is generally longer than that between the rising or falling edge of the input pulse and the falling edge of the output pulse.
Therefore, conventional waveform shaping circuits such as described above have been disadvantageous in that the square output pulse cannot be produced at the desired timing.
It has been possible to eliminate the disadvantage as described above by using a constant current circuit with each of the charging and discharging circuits for the capacitor. However, this measure has increased a change in the pulse width of the output pulse relative to a variation in source voltage.
Accordingly, it is an object of the present invention to provide a new and improved waveform shaping circuit for producing an output square pulse having a pulse width on each side of a rising or a falling edge of a square input pulse which is capable of being selected at will and is free from any change relative to a variation in the source voltage.